1. The Field of the Invention
The present invention relates to an electric rotating machine used for step drive of various kinds of devices including OA machinery such as printers. In particular the present invention relates to an improvement of a three-phase toroidal coil type permanent magnet electric rotating machine that facilitates synchronized rotation and a brushless construction.
2. Prior Art
A permanent magnet stepping motor (electric rotating machine) consists of a permanent magnet rotor having M pairs of N-poles and S-poles that are alternatively arranged in the circumferential direction around a rotor axis and a stator arranged around the permanent magnet rotor with an air gap.
A stator of a three-phase toroidal coil type stepping motor consists of three stator units stacked in an axial direction.
Each stator unit has a pair of stator magnetic poles, each of which is made of magnetic material to have M pieces of claw poles extending in the axial direction, and a toroidal coil that is sandwiched between the stator magnetic poles. The stator magnetic poles are arranged such that the claw poles of the respective stator magnetic poles are in mesh with each other inside the toroidal coil.
The toroidal coil of each stator unit has two terminals and a driving circuit excites the toroidal coils of the respective phase in turn with six terminal feeding.
In the following description, the phases of the stator units are called U-phase, V-phase and W-phase in this order from an output side of the rotor axis. The claw poles of the stator magnetic pole at the output side of the V-phase have a phase difference of 60/M degrees (mechanical angle) with respect to that of the U-phase. The claw poles of the stator magnetic poles at the output side of the W-phase have a phase difference of 60/M degrees (mechanical angle) with respect to that of the V-phase. In the same manner, the claw poles of the stator magnetic poles at the opposite side have phase differences of 60/M degrees (mechanical angle) with respect to that of the adjacent phases, respectively. A phase difference between the claw poles of the stator magnetic pole at the output side and that at the opposite side of the same phase is 180/M degrees (mechanical angle).
Under the construction, when the permanent magnet rotor is rotated by external force, the toroidal coils generate speed electromagnetic force (referred to as xe2x80x9cEMPxe2x80x9d) due to interlinkage with magnetic flux of the permanent magnets. Phase difference between EMF generated in the toroidal coils of two of three phases is 60 degrees (electrical angle).
The circuit for driving the toroidal coils of the three phases includes three bridge circuits, each of which is provided with a first pair of transistors that feed a current through a toroidal coil in a predetermined direction and a second pair of transistors that feed a current through the toroidal coil in the opposite direction. That is, the driving circuit includes twelve transistors in total.
When the driving circuit drives the above-mentioned three-phase toroidal coil type stepping motor, the circuit feeds a current in a positive direction through the toroidal coils of U, V and W phases in sequence in first three steps, and then the circuit feeds a current in a negative direction through the coils in sequence in later three steps.
This rotates the permanent magnet rotor by 60 degrees (electrical angle) in each step. That is, the rotor rotates 360 degrees by six steps.
The motor can continue rotating by repeating these six exciting steps.
However, there is a defect that the above-described driving circuit requires a large number of transistors.
In order to decrease the number of transistor, it is effective to employ a three-terminal star connection, in which one side terminals of the respective toroidal coils of the three phases are connected at a neutral point, or a three-terminal delta connection, in which the toroidal coils of the three phases are connected in a loop.
However, there is a problem that the above-mentioned conventional three-phase toroidal coil type stepping motor cannot continuously rotate in one direction with a star connection or a delta connection.
The problem is explained with reference to FIG. 9. As shown in FIG. 9, 11a and 11b denote claw poles formed on the stator unit of U-phase, 12a, 12b, 13a and 13b denote claw poles formed on the stator units of V- and W-phases, respectively, and 16 denotes the rotor.
The claw poles of the respective stator units are excited as shown in FIG. 9 when the coils of the three phases with delta connection are excited in the same manner as FIG. 5A described below.
As described above, since the claw poles 11a, 12a and 13a at the same side of the respective phases are deviated by 60/M degrees in the mechanical angle, the rotor rotates by 60/M degrees in the mechanical angle when the toroidal coils of the U-, V- and W-phases are sequentially excited with a single-phase excitation. However, if the toroidal coils of the three phases are simultaneously excited with the delta connection, the excitation sequence shown in FIGS. 5B, 5C, 6A, 6B and 6C described below cannot form a magnetic field for rotating the rotor by 60 degrees a step in the electrical angle, which interferes with the continuous rotation of the rotor 16 in one direction.
Further since the phase differences among the EMF""s of the phases detected from the three terminals are 60 degrees, the rotation angle is related to a torque, which causes torque pulsation. This reduces and changes the torque of the motor, which rises problems of generation of noise and reducing of a positioning accuracy of an object to be driven.
An object of the present invention is to solve the above described problems of the conventional motor and to provide an improved three-phase toroidal coil type permanent magnet electric rotating machine that is able to prevent torque pulsation even if a star connection or a delta connection is employed to reduce a number of transistors.
A further object of the present invention is to provide an improved three-phase toroidal coil type permanent magnet electric rotating machine that enables closed-loop control through the addition of a position sensor that detect the rotational position of a rotor.
According to a first aspect of the present invention, a three-phase toroidal coil type electric rotating machine comprises a permanent magnet rotor that comprises a cylindrical permanent magnet having M pairs of N-poles and S-poles that are alternatively arranged around a rotor axis, and a stator that comprises three stator units stacked in an axial direction with an air gap with respect to the external wall of the cylindrical permanent magnet, each of which has a pair of stator magnetic poles and a toroidal coil that is sandwiched between the stator magnetic poles, each of the stator magnetic poles being made of magnetic material to have M pieces of claw poles extending in the axial direction, the stator magnetic poles of each stator unit being arranged such that the claw poles of the respective stator magnetic poles are in mesh with each other inside the toroidal coil, wherein terminals of the toroidal coils mounted on the three stator units are connected as a star connection or a delta connection to be three-terminal feeding, wherein flux linkage of each phase formed by the cylindrical permanent magnet is an independent flux linkage that is made up of the magnetic flux from the pole opposite to the claw poles of the corresponding phase only, and wherein the positional relationship between the claw poles of the stator units and the cylindrical permanent magnet is determined such that the phase differences among speed electromotive forces caused in the toroidal coils of the respective phases due to the independent flux linkages are 120 degrees (electrical angle) between any two of the three terminals.
According to a second aspect of the present invention, the toroidal coils are controlled so as to feed the same electric currents through two of three toroidal coils of the stator units.
According to a third aspect of the present invention, an electric rotating machine comprises a permanent magnet rotor that comprises a cylindrical permanent magnet having M pairs of N-poles and S-poles that are alternatively arranged around a rotor axis, and a stator that comprises three stator units stacked in an axial direction with an air gap with respect to the external wall of the cylindrical permanent magnet, each of which has a pair of stator magnetic poles and a toroidal coil that is sandwiched between the stator magnetic poles, each of the stator magnetic poles being made of magnetic material to have M pieces of claw poles extending in the axial direction, the stator magnetic poles of each stator unit being arranged such that the claw poles of the respective stator magnetic poles are in mesh with each other inside the toroidal coil, wherein terminals of the toroidal coils mounted on the three stator units are connected as a delta connection to be three-terminal feeding, wherein flux linkage of each phase formed by the cylindrical permanent magnet is an independent flux linkage that is made up of the magnetic flux from the pole opposite to the claw poles of the corresponding phase only, and wherein the toroidal coils of the respective phases are excited such that the direction of the strong axial magnetomotive force of one phase is usually opposite to that of the weak axial magnetomotive forces of the remaining two phases when voltage is applied to two terminals with a three-phase excitation.
According to a fourth aspect of the present invention, the cylindrical permanent magnet is a polar anisotropic magnet that is anisotropic in the circumferential direction to be divided into the number of poles.
According to a fifth aspect of the present invention, each of the claw poles is formed trapezoidal shape with a tapered tip.
According to a sixth aspect of the present invention, an electric rotating machine further comprises a magnetometric sensor that detects variation of the magnetic field due to the rotation of the permanent magnet rotor, a first switching element group that consists of three switching elements connected between a plus terminal of a power source and the terminals of the three toroidal coils, respectively, and a second switching element group that consists of three switching elements connected between a minus terminal of the power source and the terminals of the three toroidal coils, respectively, wherein the first and second switching element groups are controlled according to the output of the magnetometric sensor such that one switching element of the first switching element group and one switching element of the second switching element group are selected or one switching element of one switching element group and two switching elements of the other switching element group are selected to be turned ON or OFF.
According to a seventh aspect of the present invention, the magnetometric sensor is arranged to face the cylindrical permanent magnet or to face a permanent magnet for sensor, which has 2M magnetic poles and rotates in synchronism with the permanent magnet rotor, with an air gap.
According to an eighth aspect of the present invention, an electric rotating machine further comprises a sensor pole made of magnetic material that is arranged to face the cylindrical permanent magnet or a permanent magnet for sensor having 2M magnetic poles that rotates in synchronism with the permanent magnet rotor with an air gap, and wherein the magnetometric sensor is arranged in the magnetic path passing through the sensor pole.
According to a ninth aspect of the present invention, the rotational position of the permanent magnet rotor is detected based on a waveform of exciting current modulated by a voltage induced in the respective toroidal coils by a magnetic field of the cylindrical permanent magnet, and wherein the toroidal coils are excited in order to produce a continuous rotation torque in the cylindrical permanent magnet based on the detected rotational position.
According to a tenth aspect of the present invention, the toroidal coils are driven by two-phase excitation with two-terminal feeding for the star connection or three-terminal feeding for the delta connection, and wherein one toroidal coil that is not excited is used as a sensor to detect the rotational position of the permanent magnet rotor by sensing speed electromotive force.